96 INNERVATION. [CHAP. XVIII. 



The idea of Boerhaave, and of Bressa, that sounds from without 

 which enter the mouth, or the sounds of one's own voice, were con- 

 ducted by the Eustachian tube to the labyrinth, is disproved by the 

 simple experiment of holding a sounding tuning-fork or a watch in 

 the open mouth, when it will be always found, if due care be taken to 

 avoid contact, that the sounds proceeding from them are not so well 

 heard as those from without, and the nearer they are brought to the 

 Eustachian tube the less distinctly are they heard. Indeed it may 

 be always noticed that persons deaf from obstruction of the Eusta- 

 chian tube hear the sounds of their own voices well. 



Of the Labyrinth. The essential part of the organ of hearing is 

 the vestibule. This is sufficiently proved by the constancy of this 

 part in the animal series, and by its central position in the most 

 complex ears, so that it is in close relation not only with the other 

 parts of the labyrinth but also with the tympanum. 



Sound is conveyed to the labyrinth in a threefold manner: 

 first, by the chain of bones; secondly, by the air in the tympanic 

 cavity ; in both these instances the external air is engaged in the 

 conduction ; and thirdly, through the bones of the head. 



Miiller has shown by a very interesting experiment that whilst 

 the air in the tympanum conducts sound to the cochlea through the 

 fenestra rotunda, the chain of bones forms a much better conductor 

 of it to the vestibule through the fenestra ovalis. He imitated the 

 structure of the tympanum by means of a glass cylinder, two inches 

 and one-third in diameter, and six inches long; to the neck of this 

 he fixed a wooden tube, the diameter of whose bore was eight lines. 

 The upper end of this tube was adapted to the mouth of the metal 

 flute-pipe of an organ, one foot in length; its lower extremity was 

 covered with a tense membrane of pig's bladder, which represented 

 the membrana tympani, the tube itself corresponding to the meatus 

 externus, and the cavity of the glass cylinder to the tympanum. 

 The lower opening of the glass cylinder was closed by a thick piece 

 of cork, in which two holes were cut equidistant from the walls of 

 the cylinder. In these holes, which represented the fenestrse, two 

 wooden tubes were fitted, whose outer openings were covered with 

 membrane. From the membrane of the upper tube to one of these 

 membranes, a rod was extended to imitate the chain of tympanic 

 bones extending between the membranes. 



The lower extremity of this apparatus (that namely which was 

 fitted with cork) was now introduced into water, to imitate the con- 

 nexion between the tympanum and the labyrinth in which sound 

 is conducted from air to liquid. Miiller, having plugged his ears, 



